1. Field of the Invention
The present invention relates to an image conversion method of performing an image conversion between binary area tonal image-to-binary area tonal image, an image conversion apparatus, and an image conversion program storage medium storing an image conversion program which causes a computer to operate as such an image conversion apparatus.
2. Description of the Related Art
Hitherto, to perform an image printing using a printing machine, prior to the printing, a printer or the like is used to create a proof image similar to a printed image obtained through the printing by the printing machine, and there is performed a prediction of the finish of the printed image obtained through the printing by the printing machine, referring to the proof image.
In the image printing using the printing machine, for example, a printing dot data generation apparatus receives multi-level data representative of a multi-level image, and the received multi-level data is subjected to a halftone dot processing so that halftone dot data representative of a binary area tonal image is generated by the printing dot data generation apparatus. The printing machine outputs the binary area tonal image on a sheet in accordance with the halftone dot data representative of the binary area tonal image thus generated.
To output a proof image by a printer, for example, a proofing dot data generation apparatus receives multi-level image that is the same as the multi-image data representative of the multi-level image to be inputted to the printing dot data generation apparatus, and the received multi-level data is subjected to a color conversion processing (which will be described later) and in addition the multi-level image subjected to the color conversion processing is subjected to a halftone dot processing is subjected so that proofing dot data representative of a binary area tonal image is generated by the proofing dot data generation apparatus. The printer outputs the binary area tonal image on a sheet in accordance with the proofing halftone dot data representative of the binary area tonal image thus generated. The binary area tonal image outputted from the printer is referred to a proof image.
To perform the color conversion, identified are color reproduction characteristic (a printing profile) describing a relation between image data and colors of the actual printed matter, which is associated with a printing machine of interest, and color reproduction characteristic (a printer profile) describing a relation between image data and colors of an image to be actually printed, which is associated with a printer, and for example, a LUT (Look Up Table) type of color association definition, which comprises a coupling of the printing profile and the printer profile, is created, so that image data for printing is converted into the proofing halftone dot data in accordance with the LUT. The printer outputs a proof image in accordance with the proofing halftone dot data thus obtained, and as a result, it is possible to obtain a proof image, of which color is extremely similar to the color of the actual printed matter to be printed in the printing machine.
However, generally, according to the proof image printed by such a printer, as compared with the binary area tonal image printed by the printing machine, dot structures each defined by angles and intervals in an arrangement of the individual halftone dots, and geometry and magnitude of the individual halftone dots, are mutually different from one another. Difference in the dot structure causes the Rosette pattern and the moiré pattern and in addition a way of occurrence of dot gain to be varied. Accordingly there is a possibility that a proof image different from the printing image in impression is obtained.
On the other hand, when the printing machine and the printer output binary area tonal images in accordance with halftone dot data representative of the same dot structure of binary area tonal image, respectively, the same dot structure can be obtained. However, with respect to the halftone dot data after conversion into the binary area tonal image once, it is impossible to apply the above-mentioned color conversion processing. Thus, there is a possibility that a proof image, which is expressed by a color different from the printing image, is obtained.